Lecture 5 Continental Drift How does drift occur The Ocean floor Continents move relative to each other Wegener was right Prior to 1950s very little was known about sea floor bathymetry shape of the sea floor surface Invention of echo sounding sonar permitted mapping of the ocean floor Velocity distance time Distance velocity x time Oceanographers were surprised to discover that The deepest parts of the ocean occur near land A mountain range runs through every ocean basin Submarine volcanoes form lines across oceans floors Sonar mapping delineated bathymetric features Abyssal plain 4 5 below sea level Mid oceans ridges Mors submarine mountain ranges peaks 2 3km below sea level Trenches near land 5km Volcanic islands Seamount guyots Fractures zones Modern views of the ocean floor reveal Mid ocean ridges Trenches Fracture zones Global Sea floor topography Oceanic Crust By 1950 we had learned much about oceanic crust Oceanic crust is covered by sediment Thickest near the continents Thinnest or absent at the mid ocean ridge Oceanic crust is mafic basalt and gabbro No granitic rocks No metamorphic rocks High heat flow characterizes the mid ocean ridge Belts of concentrated subsea earthquakes were found Parts of oceanic fracture zones Mid ocean axes Deep ocean trenches The earthquakes were limited to Parts of oceanic fracture zones Mid ocean ridge axes Deep ocean trenches Sea Floor Spreading In 1960 Harry Hess published his essay in Geopoetry He called his theory sea floor spreading Upwelling mantle erupts at the mid ocean ridges New crust moves away from ridges gathering sediment At trenches the sea floor dives back into the mantle Provided a potential mechanism for continental drift Magnetic Anomalies Strength of Earth s magnetic field can be measured by magnetometer At any given location the magnetic field includes two parts One created by the main dipole of the Earth Another created by the magnetism of near surface rock Anomaly difference between the expected strength of the Earth s main field at a certain location and the actual measured strength of the magnetic field at that location Or magnetic anomalies Towed magnetometers measure ocean crust Magnetic anomaly oscillates perpendicular to the MOR Anomalies And are linear belts that parallel MOR Magnetic Reversals Layered lava flows reveal reversals in polarity When reversed the north magnetic pole is near the south geographic pole Reversals are geologically rapid Radioactivity permits rock age dating A geomagnetic reversal time scale has been assembled Reversals occur every 500 700 ka 171 are known since the end Cretaceous Magnetic Reversal Chronology Reversals do not occur regularly The lengths of different polarity chrons i e time intervals between reversals are different Sea floor spreading Proof Polarity reversals explain magnetic anomalies Positive anomalies crust with normal polarity Negative anomalies crust with reversed polarity Magnetic anomalies are symmetric across the MOR Magnetic anomalies mimic layered lava flows Magnetic stripes form as lava cools at a MOR Ocean crust spreads away from MOR Reversals are recorded with cooled lava Sea Floor spreading is the mechanism of continental drift Sea Floor Spreading Age of oceanic crust increases away from the MOR Ages are mirror images across the MOR Sediments atop oceanic basalts become thicker away from the MOR Lower most oldest layers become progressively older away from the MOR Sea Floor Spreading supported by Global earthquake belts Sea Floor topography oceanic transform fault motion Linear magnetic anomalies Young age of oceanic crust Thickness and ages distribution of sediments atop oceanic basalts
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